Add note in the readme about support of earlyBootSet

packages/ember-auto-import/README.md

@@ -126,7 +126,7 @@ let app = new EmberApp(defaults, {
 Supported Options
 
 - `alias`: _object_, Map from imported names to substitute names that will be imported instead. This is a prefix match by default. To opt out of prefix-matching and only match exactly, add a `$` suffix to the pattern.
-- `earlyBootSet`: _function, returning an array of strings_, defaults to undefined, but when used, the function will receive the default earlyBootSet, which is a list of common modules found at the source of rare timing / package / ordering issues in the compatibility / cross-communication between requirejs and webpack. This is a temporary escape hatch to allow non-embroider apps to consume v2 addons at any place in their dependency graph to help ease transitioning to embroider as this problem doesn't occur once an app is using embroider. See [issue#504](https://github.com/ef4/ember-auto-import/issues/504) for details.  Note that if any modules listed here belong to v2 addons, they will be removed from the set. To opt out of default behavior, return an empty array.
+- `earlyBootSet`: _function, returning an array of strings_, (only supported on ember-source >= 3.27.0) defaults to undefined, but when used, the function will receive the default earlyBootSet, which is a list of common modules found at the source of rare timing / package / ordering issues in the compatibility / cross-communication between requirejs and webpack. This is a temporary escape hatch to allow non-embroider apps to consume v2 addons at any place in their dependency graph to help ease transitioning to embroider as this problem doesn't occur once an app is using embroider. See [issue#504](https://github.com/ef4/ember-auto-import/issues/504) for details.  Note that if any modules listed here belong to v2 addons, they will be removed from the set. To opt out of default behavior, return an empty array.
 - `exclude`: _list of strings, defaults to []_. Packages in this list will be ignored by ember-auto-import. Can be helpful if the package is already included another way (like a shim from some other Ember addon).
 - `forbidEval`: _boolean_, defaults to false. We use `eval` in development by default (because that is the fastest way to provide sourcemaps). If you need to comply with a strict Content Security Policy (CSP), you can set `forbidEval: true`. You will still get sourcemaps, they will just use a slower implementation.
 - `insertScriptsAt`: _string_, defaults to undefined. Optionally allows you to take manual control over where ember-auto-import's generated `<script>` tags will be inserted into your HTML and what attributes they will have. See "Customizing HTML Insertion" below.